This invention relates generally to orthodontic devices. More specifically, this invention relates to lingual orthodontic brackets.
Two primary types of orthodontic brackets have been used in the prior art, namely, labial and lingual brackets. As the name implies, labial brackets are positioned on the labial surfaces of a person's teeth. The most conventional type of labial bracket is a symmetric dual-wing bracket as shown in FIGS. 1 and 2. Referring to FIGS. 1 and 2, this bracket 1 has a bonding pad 4, a base member 6, and two opposing wings 12. An archwire slot 10 is located between the two wings 12 and receives an archwire 8 at a right angle to the plane of the bonding pad 4. After the archwire 8 is in place within the archwire slot 10, an O-ring 14 is placed around one of the wings 12 and then stretched over the opposing wing 12 to retain the archwire 8 in place. This single tie configuration allows the elastic tensile forces of both halves of the O-ring 14 to symmetrically oppose a force (represented by arrow 16) tending to remove the archwire 8 from the archwire slot 10. O-rings 14 must typically be replaced at least every six weeks because their elastomeric material becomes degraded over time as a result of the chemical environment of the mouth. When it is time to replace the O-rings 14, the O-rings 14 can be removed and replaced one at a time by simply disengaging the end portions of the O-ring 14 from the wings 12. The conventional labial bracket 1 is therefore simple to use.
Lingual brackets are attached to the lingual surfaces of a person's teeth. Lingual brackets are desirable because they are not readily noticeable to an outside viewer. Unlike the labial brackets described above, however, the conventional lingual brackets, as shown in FIGS. 3, 3A, 4, and 4A, are generally difficult or inefficient to use. Referring to FIGS. 3, 3A, 4, and 4A, a conventional lingual bracket 20 is asymmetric and comprises a bonding pad 24, a base member 36, and opposing occlusal (or incisal) and gingival wings 32 and 38, respectively. The conventional lingual bracket also includes an archwire slot 30 located between the two wings 32 and 38 and angled toward the gingival wing 38. The archwire slot 30 has an entrance in the direction of the gingival wing, opposing first and second walls, and a base located opposite the entrance in the direction of the base member 36. Examples are shown in U.S. Pat. Nos. 4,337,037 and 4,669,981 to Kurz. The gingival wing 38 can include an elongated extension with a knob end as shown in Kurz '981 or can be T-shaped to serve as a hook for inter-maxillary elastics.
FIGS. 3 and 4 show the most typical tying arrangement of the conventional lingual bracket, i.e., double-tied. Referring to FIGS. 3 and 4, an O-ring 34 is generally double-tied around the archwire 26 of the conventional lingual bracket 20. In a double-tie configuration, the O-ring 34 must first be placed around the base 36. Only after the O-ring 34 is positioned around the base 36 is the archwire 26 inserted into the archwire. slot 30 in a direction at an acute angle to the plane of the bonding pad 24. A second end portion of the O-ring 34, located between the gingival wing 38 and the bonding pad 24, is then stretched over the gingival wing 38, around the archwire 26 and back over the occlusal wing 32, forming a substantially U-shaped O-ring configuration, as viewed from a side view. The tensile forces of both halves of the O-ring 34 retain the archwire 26 within the archwire slot 30 by nearly symmetrically opposing a force (represented by arrow 22) tending to disengage the archwire 26 therefrom.
Unfortunately, there are disadvantages with the double tied lingual bracket of the prior art. First of all, as with labial brackets, the O-rings of lingual brackets must be replaced frequently. Unlike the single tied labial brackets, however, the process for replacing the O-ring on a double tied lingual bracket is complex. To replace the O-ring, the second end portion of the O-ring must first be stretched back over the occlusal and gingival wings such that the O-ring surrounds only the base member. Then, the archwire must be removed from the archwire slot. Only then can the old O-ring be completely removed from the bracket. Finally, a new O-ring must be attached using the steps described above for attaching the original O-ring. This process must be repeated for each of the lingual brackets needing O-ring replacement.
Another serious disadvantage with the use of double tied conventional lingual brackets is that the O-ring must be stretched much further than that of the labial bracket. Excessive stretching further reduces the life of the O-ring. Furthermore, to accommodate this amount of stretching, the double tied lingual bracket O-ring must be highly elastic. This increased elasticity adversely affects its tensile strength, and hence its ability to oppose forces that remove the archwire from its slot.
To avoid some of these disadvantages associated with double tying the O-ring of the conventional lingual brackets, a single tie configuration for the same brackets has been attempted. Although it is possible to single tie the archwire into the archwire slot of the conventional lingual bracket, using a single tie configuration with the conventional lingual bracket is disadvantageous because it results in a very weak retaining force. FIGS. 3A and 4A show a conventional lingual bracket using a single tie configuration. As shown in FIGS. 3A and 4A, in a conventional lingual bracket 20 with a single tie configuration, the O-ring 34 is placed over the gingival wing 38 and then stretched over the occlusal wing 32. In this configuration, a force (represented by arrow 22) acting to disengage the archwire 26 from the archwire slot 30 is only weakly opposed by the O-ring 34 for several reasons. First of all, only the portion of the O-ring 34 extending from the occlusal wing 32 has a tensile force which directly opposes the archwire's 26 movement away from the slot 30. The other portion of the O-ring 34, which is attached around the gingival wing 38 at approximately a right angle to the first portion, is free to slide along the gingival wing 38. It does not, therefore, provide any significant retaining force. Further disadvantageous is the fact that the O-rings used to single tie the archwire into the archwire slot of the conventional lingual bracket must be replaced more frequently than those used in a double tie configuration because they only have one portion of the O-ring that directly opposes the force tending to remove the archwire from the archwire slot. These drawbacks associated with conventional lingual brackets have made their use much less desirable than the labial brackets, despite the fact that many people prefer them aesthetically.
It should be noted that all of the dual wing prior art brackets described above rely solely on tensile properties of the O-ring to retain the archwire within the archwire slot. Neither prior art bracket contemplates the use of other properties of the O-ring which might aid in retaining the archwire within its slot.
The industry has been unable to provide to the orthodontic profession a lingual bracket which allows easy O-ring attachment and replacement while providing sufficient retaining strength. Furthermore, the industry has relied solely on the tensile elastic properties of O-rings to retain the archwire within the archwire slot of both lingual and labial brackets. What the industry needs, therefore, is a lingual bracket which facilitates easy attachment and replacement of O-rings without compromising retaining strength. The profession would also be benefited by a method of retaining an archwire within the archwire slot of a lingual bracket which uses a more stretchresistant O-ring.